The shield of the cable is "supposed" to be grounded by the cable company, probably to the block inside the locked box. I'm not sure simply adding a suppressor to the line is going to accomplish anything unless the cable installer didn't connect the ground like he was supposed to.

Buy one of the suppressor boxes that includes protection for the AC line, telephone line, and coaxial cable feeds. I currently use TrippLite boxes and they come with a guarantee of protection for attached devices - many such surge protectors include that. I had two damaging strikes prior to improving my home lightning protection and everything attached to the TrippLites survived just fine.

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